The present invention relates to an electrophotographic copier, laser printer, facsimile apparatus or similar image forming apparatus. More particularly, the present invention is concerned with a belt transfer device for transferring a toner image from an image carrier to a recording medium and conveying the medium while electrostatically retaining it thereon and including a belt passed over a drive roller and a driven roller, charge applying means for applying a charge to the belt, a discharging member for discharging the belt, and a cleaning device for cleaning the surface of the belt.
In a belt transfer device of the type described, a bias for image transfer is continuously applied to a bias roller associated with a belt during copy mode operation. The bias deposits a charge opposite in polarity to the charge of toner deposited on a photoconductive drum on the belt having a preselected resistance. As a result, a toner image is transferred from the drum to a paper or similar recording medium. After the image transfer, cleaning means cleans the belt in order to remove the toner and impurities including paper dust.
The above cleaning means is generally implemented by a blade which is simple in configuration and low cost, as in the other image forming means. However, a problem is that a coating material having a small coefficient of friction and highly extendible cannot be easily selected for the blade in combination with the belt having elasticity. A hard material having a small coefficient of friction would form cracks in the surface of the belt and would allow toner to enter the cracks, resulting in defective cleaning. Moreover, highly extendible materials in general have great coefficients of friction and are apt to cause the blade to be turned up or to obstruct the separation of toner, also resulting in defective cleaning.
In light of the above, a current trend in the imaging art is toward a bias roller or similar electrostatic cleaning means replacing the blade. However, allocating a particular bias power source to each of electrostatic cleaning and image transfer is undesirable from the cost and space standpoint. To solve this problem, Japanese Patent Laid-Open Publication No. 8-76608, for example, teaches a system allowing electrostatic cleaning and image transfer to share a single power source. This system, however, brings about another problem that toner directly deposited on the belt is apt to partly remain even after cleaning unless due consideration is given to the OFF/OFF timing in relation to the shared bias power source.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 10-3222.